Although computers running Windows Vista are significantly less likely to be infected with attack code than machines running Windows XP, the newer operating system continues to be threatened by Microsoft Corp.'s own ActiveX browser plug-in technology, according to a report issued Monday by the company.

In the most recent installment of its twice-yearly security intelligence report, Microsoft said that PCs running Windows XP Service Pack 2 (SP2) were more than three times as likely to be infected with malware as computers running Windows Vista SP1. Machines powered by the newest XP security update, SP3, meanwhile, were more than twice as likely to be infected.

According to Microsoft, in the six months from January to June, its Malicious Software Removal Tool (MSRT) cleaned malware from just three Vista SP1 machines per thousand times the tool was run. Meanwhile, during the same period, MSRT found and wiped malicious code from 10 Windows XP SP2 systems and eight XP SP3 PCs per thousand executions. Microsoft updates and automatically redistributes the software tool to Windows users each month on Patch Tuesday.

"Our security development processes do pay off," said George Stathakopoulos, the general manager of Microsoft's product security and security engineering group, referring to work the company has put into writing more secure code for its newer software, including Vista. "We're fairly happy where Microsoft is," Stathakopoulos continued, "but ecosystemwide, we still have a problem."

That's evident from Microsoft's data for the past six months. During that time, while half of the top 10 browser-based attacks against Windows XP machines relied on vulnerabilities in Microsoft's own software, none of the top 10 attacks against Vista systems did. Instead, the overwhelming majority of the browser attacks targeting Vista leveraged bugs in third-party companies' ActiveX controls.

Vulnerabilities in ActiveX, the Microsoft technology used to create add-ins for Internet Explorer (IE), accounted for eight of the top 10 browser-based attacks against Vista in the first half of 2008. A ninth vulnerability could be exploited via ActiveX, among other means.

Two of the eight vulnerability ActiveX controls were part of RealNetworks Inc.'s RealPlayer media player plug-in; another was part of Apple Inc.'s QuickTime player. Both vendors have had to repeatedly patch their programs this year. Apple alone has patched a total of 30 QuickTime vulnerabilities in five updates in 2008.

Microsoft's numbers echo data collected by Symantec Corp. for the latter half of 2007, when ActiveX bugs accounted for 79% of all those discovered in browser plug-ins during that period.